The invention concerns a security element in the form of a multi-layer film body having a volume hologram layer with two different items of optical information and a process for the production of such a security element.
Holograms are used as security elements for protecting security documents such as banknotes, money replacement means, credit cards, passports or ID documents as well as for product security. Mass-produced articles frequently use surface holograms which make it possible to achieve interesting, optically variable effects, for example motion effects, and which are distinguished by a high level of light strength.
Volume holograms, also referred to as white light holograms or Lippmann holograms, in contrast to surface holograms, are based on light diffraction at the Bragg planes of a transparent layer which has local refractive index differences.
A security element having a volume hologram and the production of such a security element is described for example in DE 10 2006 016 139 A1. A surface relief is used as the master for production of a multi-layer body which includes a volume hologram. The front side of the master is brought into contact with the photosensitive layer of the multi-layer body in which the volume hologram is to be recorded, directly or with the interposition of a transparent optical medium. The master is then exposed with coherent light, in which case an interference pattern is produced by superimpositioning of the light radiated on to the master and the light diffracted by the master, the interference pattern being recorded in the photosensitive layer as a volume hologram. The volume hologram which is introduced into the photosensitive layer in that way is then fixed by hardening of the photosensitive layer. In that respect, two or more separate items of image information can be written into the photosensitive layer by virtue of the master being of a special configuration.
In addition EP 1 187 728 B1 describes laminating two volume hologram layers on to each other, in which layers items of image information have been written by means of different holographic recording processes. That gives the viewer an overall impression which is composed of the items of image information of the two volume hologram layers. In that case however there is the disadvantage that the security elements which are made up of two volume hologram layers are relatively thick and it is difficult for the two volume holograms to be laminated on to each other in accurately fitting relationship.
It is known that organic solvents which are brought into contact with volume holograms can increase the degree of diffraction effectiveness and can influence chromaticity. Diffusion of organic molecules into the volume hologram can cause swelling and thus a change in the spacing of the holographic lattice planes, which in particular for reflection volume holograms results in a colour enhancement effect (U.S. Pat. No. 5,415,950). It will be noted however that it is not possible with solvents to swell only partial parts of the hologram which represent new visible information. Expansion agents are required for that purpose, the diffusion properties of which can be varied in locally specifically targeted fashion.
WO 2007/131496 describes a process in which a holographic photofilm is brought into contact with a light-hardenable expansion agent which is subjected to mask exposure so that differentiated expansion of the photofilm occurs due to differentiated diffusion of the expansion agent in the photofilm, wherein expansion is terminated after a defined period of time by complete exposure of the expansion agent.
U.S. Pat. No. 5,182,180 describes a process in which a partially exposed diffusion film is applied to a volume hologram, thereby causing local swelling of the volume hologram.